The invention relates to a printing press having an inking unit, especially a short inking unit, and to a method of operating an inking unit.
Such printing presses and methods have become known heretofore. A conventional printing press comprises an inking unit, for example an anilox inking unit, having a screen roller for metering a printing medium, for example ink or varnish, to be transferred to an ink applicator roller cooperating with the screen roller. The screen roller is formed in the circumferential surface thereof with a pattern of depressions, such as individual cells or lines, for example. The printing medium is transferred from the screen roller to a printing form, for example a plate cylinder, and from the latter to printing material. Due to the pattern of the screen roller, full-tone areas of a printed image applied to the printing material are not closed, but rather one can detect instead the fine screen roller structure, which is not desired.
In order to prevent the pattern of the screen roller from being detected on the printed image, additional rider rollers, for example, are used on the ink applicator roller, for the purpose of distributing this pattern. Disadvantages here are the costly construction of the printing unit and the fact that the additional rollers cause the inking unit to be no longer free of ghosting. Furthermore, it has become known heretofore to form the pattern of the screen roller so fine that it is no longer perceived as disruptive by the eye of an observer. However, as the pattern becomes finer, the amount of ink that is transferable, i.e., the scooping volume of the depressions formed in the screen roller and, therewith, also the density of the printed image applied to the printing material decreases. As a result, the cleaning of the screen roller is very difficult. Furthermore, the published German Patent Document DE 44 31 464 A1 discloses a device which permits a differential circumferential speed between screen roller and ink applicator roller in order to blur the printing-medium pattern on the ink applicator roller. It has been shown, however, that the printing medium film on the ink applicator roller cannot thereby be evened out in the desired manner and that for differential circumferential speeds of greater than about 5%, the printing medium density simply decreases, while the pattern in the printed image continues to remain visible.
It is accordingly an object of the invention to provide a printing press having an inking unit with which a most possibly closed full-tone area can be printed, and which preferably has a simple and cost-effective construction. A further object of the invention is to provide a method of operating an inking unit wherein a most possibly closed full-tone area is produced preferably simply.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a printing press, comprising an inking unit provided with a screen roller having, on a circumference thereof, a pattern formed of depressions fillable with a printing medium, another roller co-operatively engageable with the screen roller, and a drive device, the screen roller and the other roller being drivingly coupled with one another so that, after each revolution of the screen roller, a pattern depicted by the printing medium on the other roller in a form of printing medium accumulations has a slight offset, in circumferential direction, with respect to a pattern depicted on the other roller during a preceding revolution of the screen roller, so that the new printing medium accumulations are positionable on the other roller in at least approximately printing-medium free gaps in, respectively, adjacent printing medium residual accumulations.
In accordance with another feature of the invention, the screen roller and the other roller are coupled with one another via at least a one-stage drive gear transmission.
In accordance with a further feature of the invention, the printing press further comprises a main drive with which the other roller is coupled, and a separate motor with which the screen roller is coupled.
In accordance with an added feature of the invention, the printing press further comprises a variable-ratio gear transmission via which the screen roller and the other roller are connected to one another.
In accordance with an additional feature of the invention, the other roller is drivable at printing-press speed, and the screen roller has a selectively increasable and decreasable circumferential speed for setting a differential circumferential speed between the screen roller and the other roller.
In accordance with yet another feature of the invention, the inking unit is a short inking unit.
In accordance with another aspect of the invention, there is provided a method of operating an inking unit having a screen roller in co-operative engagement with another roller, the screen roller having, on the circumference thereof, a pattern formed of depressions fillable with a printing medium, which comprises rotating the screen roller and the other roller in a manner coordinated with one another so that after each revolution of the screen roller, the pattern depicted by the printing medium on the other roller in a form of printing medium accumulations has a slight offset in circumferential direction with respect to a pattern depicted on the other roller during a preceding revolution of the screen roller so that new printing medium accumulations are positionable on the other roller in at least approximately printing-medium free gaps in respectively adjacent printing medium residual accumulations.
In accordance with a concomitant aspect of the invention, the method is for operating a short inking unit.
Thus, the inking unit, and more specifically, a short inking unit, has a screen roller, for example an anilox roller, which has, on the circumference thereof, a pattern formed of depressions. The depressions can be filled with a printing medium, for example liquid ink or varnish. The screen roller is in contact with another roller, for example an ink applicator roller, to which the printing medium in the depressions is applied. The printing press is distinguished by the fact that the screen roller and the other roller have a drive connection with one another so that after each revolution of the screen roller, the pattern depicted by the printing medium on the other roller in the form of printing medium accumulations has a slight offset in the circumferential direction with respect to a pattern depicted on the other roller during a preceding revolution of the screen roller so that the new printing medium accumulations can be positioned on the other roller in the printing-medium free gap or approximately printing-medium free gaps in respectively adjacent printing medium residual accumulations. The printing medium in the depressions is therefore transferred to the other roller in the nip formed between the screen roller and the other roller and, on the circumferential surface of the latter, forms small printing medium accumulations which, in the circumferential direction of the other roller, are at a constant distance from one another corresponding to the pattern of depressions. In the course of a rotation of the other roller, after part of the accumulated printing medium has been discharged by splitting to a printing form co-operating with the other roller, for example a plate cylinder, printing medium residual accumulations having a volume and a height which have been reduced remain on the other roller. Between these printing medium residual accumulations are virtually printing-medium free gaps. The rolling of the screen roller and the other roller on one another is set so that when the printing-medium free gaps on the other roller are moved past the screen roller, the next, fresh printing medium accumulations are placed by the latter onto the other roller exactly in these gaps between the printing medium residual accumulations. In order to provide the gaps between the printing medium accumulations with printing medium, as distinct from heretofore known methods, these accumulations are therefore not blurred; instead, the virtually printing-medium free gaps between the printing medium residual accumulations on the other roller are filled in a controlled manner with fresh printing medium accumulations. As a result, a printing medium surface relief is produced on the other roller, the printing medium layer thickness thereof fluctuating comparatively only slightly, as a result of which the print quality is increased and continuous inking of a printed full-tone area is realizable. The pattern formed by the depressions on the screen roller can therefore not be detected in the printed image applied to a printing material.
The size of the offset between the fresh printing medium accumulation and the printing medium residual accumulation depends upon the distance from one another of the depressions provided on the screen roller. If, for example, the screen roller has a cell or line pattern, then, for example up to 50 lines can be provided in one centimeter. Here, the distance between two adjacent lines can lie in a range from a few hundredths of a millimeter up to a very few tenths of a millimeter.
In the case of likewise suitable screen roller engraving, the screen roller pattern is formed by cells or lines, of which more than 100 can be arranged on one centimeter. It becomes clear that the offset must be extremely precise in order that the new printing medium accumulations are placed exactly in the printing-medium free gaps or approximately printing-medium free gaps of respectively adjacent printing medium residual accumulations on the other roller.
In an advantageous exemplary embodiment of the printing press, provision is made for the screen roller and the other roller to be coupled with one another via a single-stage or multi-stage drive gear transmission. In a particularly advantageous variation in construction, the overall transmission ratio xe2x80x9cixe2x80x9d of the drive gear transmission is selected, and the diameter dR of the screen roller and the diameter dW of the other roller are coordinated with one another in such a way that the circumferential speed difference is zero, i.e., the screen roller and the other roller roll on one another without slip, but nevertheless, following each revolution of the screen roller, the pattern depicted on the other roller having a slight offset in the circumferential direction so that the new printing medium accumulations are placed on the other roller in the virtually printing-medium free gaps of respectively adjacent printing medium residual accumulations. This is realized, for example, by the transmission ratio being 1:1.999, the diameter dW of the other roller being 200 mm, and the diameter dR of the screen roller being 100.05 mm. The diameter dW of the other roller is preferably the same as the diameter of a plate cylinder in contact with the other roller.
Furthermore, the method of the invention provides for the screen roller and the other roller to rotate on one another in a manner coordinated so that after each revolution of the screen roller, the pattern depicted by the printing medium on the other roller in the form of printing medium accumulations has a slight offset in the circumferential direction with respect to a pattern depicted on the other roller during a preceding revolution of the screen roller, so that the new printing medium accumulations can be positioned on the other roller in the printing-medium free gaps or approximately printing-medium free gaps in respectively adjacent printing medium residual accumulations. The offset can be implemented both in the direction of rotation of the rollers and counter to the direction of rotation of the rollers.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a printing press having an inking unit, and a method of operating an inking unit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.